Parallelization is always limited by the overhead for communication ...

from MCE::Map

The time for mce_map below also includes the time for data exchanges between the manager and worker processes.

Please try

my @squared_big_list = mce_map_s { $_ * $_ } 1,10000000;
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Even faster is mce_map_s; useful when input data is a range of numbers. Workers generate sequences mathematically among themselves without any interaction from the manager process.

(untested)

update

to elaborate more on what AnoMonk said here:

$_ * $_ is so fast that even a slight communication overhead counts. The more complex the calculation, the less significant will the communication become.

The relation matters.